The invention relates to polyethylene. More particularly, the invention relates to polyethylene that has improved rheological properties and thermal processability.
Single-site catalysts often produce olefin polymers having relatively low molecular weight and narrow molecular weight distributions. The uniformity of molecular weight distribution, although improving tensile strength and other physical properties of polymer products, makes the thermal processing more difficult. Olefin polymers having low molecular weights also cause problems in thermal processing because they have low viscosity and low melt strength at low shear rates.
In contrast, polyethylenes made with Ziegler catalysts have relatively high molecular weight and broad molecular weight distribution. They have high melt strength but low melt index. It is desirable to produce a polyethylene having a combination of the properties of single-site polyethylene and Ziegler polyethylene. U.S. Pat. No. 6,127,484, for example, teaches a multiple-zone, multiple-catalyst process for making polyethylene. A single-site catalyst is used in a first reaction zone to produce a polymer having a relatively low molecular weight and a Ziegler catalyst is used in a second zone to make a polymer having a relatively high molecular weight. The two polymers are mixed to give a polymer having a broad molecular weight distribution and improved processability.
It is also known that increasing long-chain branching can improve processability of polyethylene made with single-site catalysts (see WO 93/08221). The existence of long-chain branching in polyethylene is particularly important for blown film extrusion and blow molding processes. However, achieving long-chain branching often requires the use of specific catalysts. For example, WO 93/08221 teaches how to increase the concentration of long-chain branches in polyethylene by using constrained-geometry single-site catalysts.
New polyethylene is needed. Ideally, the polyethylene would have both high melt indexes like single-site polyethylene and high melt strength like Ziegler polyethylene.
The invention is a polyethylene having improved rheological properties. The polyethylene has a melt index (MI2) from about 0.01 to about 50 dg/min and a melt strength (xcex7) that satisfies MI2xc3x97xcex7xe2x89xa73.5. The polyethylene can be made by using an azaborolinyl-containing single-site catalyst in the presence of hydrogen, where the hydrogen consumption is controlled to be less than about 30 wt %.
The polyethylene of the invention has unique rheological properties. It has a melt index MI2 from about 0.01 dg/min to about 50 dg/min. Preferably, the MI2 is from about 0.05 dg/min to about 10 dg/min. MI2 is measured at 190xc2x0 C. under 2.16 kilograms of pressure, according to ASTM D-1238.
The polyethylene of the invention has a melt strength xcex7 that satisfies MI2xc3x97xcex7xe2x89xa73.5. Preferably, xcex7 satisfies MI2xc3x97xcex7xe2x89xa74.0. More preferably, xcex7 satisfies MI2xc3x97xcex7xe2x89xa75.0.
Melt strength xcex7 is the ability of a polymer melt, in an extensional-type deformation, to withstand disturbances that tend to destabilize the process. For example, melt strength in a blown film process usually refers to the bubble stability. Melt strength is commonly characterized by low shear viscosity. See M. H. Naitove and J. H. Schut, Plastics Technology, (October 1993) 41-44. Low shear viscosity can be conveniently obtained by dynamic viscoelastic measurements. In this test, an oscillatory shear deformation with a frequency xcfx89 and a strain xcex3 is imposed on the polymer melt and the resultant stress response is measured. The ratio of the stress to the strain is the complex modulus, G*, from which the complex viscosity, xcex7*, is obtained:
xcex7*=G*/xcfx89
See J. M. Dealy and K. F. Wissbrun, Melt Rheology and It Role in Plastics Processing, Van Nostrand Reinhold, New York (1990). Melt strength is defined as:
xcex7=xcex7*xc3x9710xe2x88x925 
The xcex7 is measured at 190xc2x0 C. and at G*=5000 dyn/cm2.
We have found that the known polyethylenes have a value of MI2xc3x97xcex7 significantly lower than that of polyethylenes of the invention. For instance, Comparative Example 5 shows that a polyethylene made with a borabenzene-based single-site catalyst has MI2xc3x97xcex7 of 3.1. Moreover, Comparative Examples 6-9 show that polyethylenes made with Ziegler catalysts have MI2xc3x97xcex7 values that are even lower.
Preferably, the polyethylene of the invention comprises less than about 15 wt % of C3 to C10 xcex1-olefin recurring units. Examples of suitable C3 to C10 xcex1-olefins are propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, and the like, and mixtures thereof. 1-Hexene and 1-butene are preferred. Incorporating long-chain xcex1-olefins reduces the density of polyethylene. The density of the polyethylene is preferably from about 0.89 g/cm3 to about 0.96 g/cm3. More preferably, the density is within the range of about 0.90 g/cm3 to about 0.94 g/cm3. Most preferably, the density is from about 0.90 g/cm3 to about 0.93 g/cm3.
The polyethylene preferably has a weight average molecular weight (Mw) from about 10,000 to about 1,500,000 and a molecular weight distribution (Mw/Mn) less than about 3.5. More preferably, the Mw is from about 30,000 to about 1,000,000. Most preferably, the Mw is from about 30,000 to about 800,000. The Mw/Mn is more preferably less than about 3.0.
The invention includes a process for making the polyethylene. The process uses an azaborolinyl-containing single-site catalyst. Preferably, the single-site catalyst is a Group 3-10 transition metal compound that contains at least one azaborolinyl ligand. Group 4 transition metal compounds are preferred. Azaborolinyl-containing single-site catalysts are known. For instance, U.S. Pat. No. 5,902,866, the teachings of which are incorporated herein by reference, teaches the preparation of azaborolinyl-containing single-site catalysts and the polymerization of an olefin by using the catalysts.
The catalyst contains other ligands. The total number of ligands satisfies the valence of the transition metal. Other suitable ligands include substituted or unsubstituted cyclopentadienyls, indenyls, fluorenyls, halides, C1-C10 alkyls, C6-C15 aryls, C7-C20 aralkyls, dialkylamino, thioether, siloxy, alkoxy, and the like, and mixtures thereof. Halides, cyclopentadienyls, and indenyls are preferred.
Examples of suitable single-site catalysts are (azaborolinyl)(indenyl)titanium dichloride, (azaborolinyl)(indenyl)zirconium dichloride, (azaborolinyl)(cyclopentadienyl)titanium dichloride, (azaborolinyl)(cyclopentadienyl)zirconium dichloride, and the like, and mixtures thereof. (Azaborolinyl)(cyclopentadienyl)zirconium dichloride is preferred.
Optionally, the catalyst is immobilized on a support. The support is preferably a porous material such as inorganic oxides and chlorides, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred supports include silica, alumina, silica-aluminas, magnesias, titanias, zirconias, magnesium chloride, and crosslinked polystyrene. Silica is most preferred.
Preferably, the support has a surface area in the range of about 10 to about 900 m2/g, a pore volume in the range of about 0.1 to about 4.0 mL/g, an average particle size in the range of about 10 to about 500 xcexcm, and an average pore diameter in the range of about 10 to about 1000 xc3x85. Supports are preferably modified by heat treatment, chemical modification, or both. For heat treatment, the support is preferably heated at a temperature from about 50xc2x0 C. to about 800xc2x0 C. More preferably, the temperature is from about 50xc2x0 C. to about 300xc2x0 C.
Suitable chemical modifiers include organoaluminum, organosilicon, organomagnesium, and organoboron compounds. Organosilicon and organoboron compounds, such as hexamethyldisilazane and triethylborane, are preferred. Suitable techniques to support a single-site catalyst are taught, for example, in U.S. Pat. No. 6,211,311, the teachings of which are incorporated herein by reference.
The catalyst is used with an activator. Activators can be combined with the catalyst and the optional support or they can be added separately to the polymerization reactor. Suitable activators include anionic compounds of boron and aluminum, trialkylborane and triarylborane compounds, and the like. Examples are lithium tetrakis(pentafluorophenyl) borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, tris(pentafluorophenyl) borane, methyl alumoxane (MAO), the like and mixtures thereof. Activators are generally used in an amount within the range of about 0.01 to about 100,000, preferably from about 0.1 to about 1,000, and most preferably from about 0.5 to about 300, moles per mole of the single-site catalyst.
The process can be performed in solution, slurry, and gas phase. Gas phase and slurry phase processes are preferred. Methods and apparatus for gas phase polymerization of ethylene with Ziegler catalysts are well known, and they are suitable for use in the process of the invention. For example, U.S. Pat. No. 5,859,157, the teachings of which are herein incorporated by reference, teaches in detail gas phase polymerization of ethylene with a Ziegler catalyst. The slurry phase polymerization is performed in an organic solvent that can disperse the catalyst and polyethylene. Suitable solvents include C4 to C10 linear, branched, and cyclic aliphatic, and C6-C12 aromatic hydrocarbons. Examples of suitable solvents are butane, hexane, cyclohexane, octane, and isobutane.
The polymerization is preferably conducted under pressure. The pressure is preferably in the range of about 100 to about 15,000 psi, more preferably from about 200 to about 5,000 psi, and most preferably from about 200 to about 2,000 psi. Generally, the higher the pressure, the more productive the process. Laboratory operations are conducted under relatively low pressure for safety reasons.
The process of the invention is performed in the presence of hydrogen. The hydrogen concentration is preferably greater than about 5 moles per mole of the catalyst. The hydrogen consumption rate, i.e., the ratio of the amount of hydrogen consumed over the amount charged, is controlled to be lower than about 30 wt %. We have surprisingly found that when the hydrogen consumption rate is too high, the polyethylene produced does not have the above-discussed characteristics.
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.